Circuits for the detection of analog and of digital signals typically involve an analog-to-digital (A/D) converter. An A/D converter changes an analog signal into a digital value. A/D converters are commonly employed to change a signal from a light sensor or from a temperature-dependent resistor into a digital signal. The digital signal can then be processed by a microprocessor.
Circuits supplying an actuator are frequently based on a digital-to-analog (D/A) converter. A digital-to-analog converter changes a digital value into an analog signal. The D/A converter commonly provides a signal and sets the position of an actuator.
The output module of a universal input/output circuit typically involves an amplifier. An amplifier is required because the impedance at the output of the D/A converter does not match the impedance at the input of the actuator. The amplifier preferably is a unity gain amplifier which amplifies an electric current without changing voltage.
The patent EP1500992B1 issued on 9 Nov. 2005 and discloses an apparatus for operating field devices of a building automation system. Field devices of building automation systems are, by way of non-limiting example, temperature sensors, humidity probes, light sensors, actuators for blinds, valve actuators etc. The apparatus of EP1500992B1 provides a universal input/output circuit that connects to various field devices.
The universal input/output circuit of EP1500992B1 includes an A/D converter to carry out measurements of temperature, humidity, air quality etc. An additional D/A converter sets the position of an actuating device in accordance with a voltage signal. A unity gain amplifier matches the output impedance of the D/A converter to the impedance of the actuator. The input/output circuit of EP1500992B1 also provides a number of switches. These switches allow for reconfiguration of the circuit such that the same universal input/output circuit can be used for reading sensors and also for driving actuators.
To minimize the number of components, the circuit sequentially obtains readings. That is, the apparatus may simultaneously connect to multiple sensors such as light sensors and thermocouples. A time frame is allocated to each sensor and the input/output circuit takes measurements from the sensors in accordance with allocated time frames. An input/output circuit adapted for sequential readings dispenses with separate A/D converters for every sensor.
Resistance measurements of field devices are generally prone to dielectric absorption. Those issues are due to the different time constants of molecular relaxation of the charge carriers on a capacitor. They may arise whenever a capacitive element is in parallel with a (resistive) sensor. An electric charge remaining on a capacitor will thus impair the resistance measurement. The parallel resistance may actually be a stray capacitance and may be caused by leads that connect to a field device.